1. Field of the Invention
The present invention relates generally to control devices for internal combustion engines and particularly to control devices for internal combustion engines having a cylinder receiving fuel directly injected thereinto.
2. Description of the Background Art
Conventionally a direct injection engine directly injecting fuel into a cylinder is commercially available. The direct injection engine includes an engine allowing fuel to be injected at an intake stroke to combust a homogeneous air fuel mixture to provide homogeneous combustion, and an engine allowing fuel to be injected at a compression stroke to combust an air fuel mixture with an air fuel ratio increased around an ignition plug to provide stratified combustion. Furthermore, there also exists a direct injection engine dividing fuel injection in two, i.e., injecting the fuel at intake and compression strokes. Such a direct injection engine can also knocks, and such knocking needs to be minimized.
Japanese Patent Laying-Open No. 2004-003429 discloses a controller that can reduce knocking caused in a stratified combustion mode of operation, without inviting misfire, poor exhaust emission and the like. As described in the publication, in the stratified combustion mode of operation when a knock detector detects that engine knocks the controller controls a knock prevention controller for the stratified combustion mode to introduce spark retard for correction as well as fuel injection retard for correction and switches the stratified combustion mode to a homogeneous combustion mode. In reducing the knocking, as controlled, when an amount or result of correcting at least one of spark retard and fuel injection retard exceeds a prescribed threshold value, the stratified combustion mode is switched to a double injection stratified combustion mode. When the detector detects that the engine operating in the double injection stratified combustion mode knocks, a knock prevention controller associated with the mode corrects at least one of a ratio between fuel injected at the intake stroke and that injected at the compression stroke, the timing of fuel injection at the intake stroke, and the timing of spark. It should be noted that the timing of fuel injection at the compression stroke is not corrected. Furthermore in the double injection stratified combustion mode if the engine knocks and the knocking is reduced as controlled, and as a result at least one of an amount or result of correcting the fuel distribution ratio, the timing of fuel injection at the intake stroke, and the timing of spark exceeds a prescribed threshold value, then the double injection stratified combustion mode is switched to a homogeneous combustion mode.
As disclosed in the publication, the knock reduction controller can introduce a spark retard in the stratified combustion mode of operation for correction to prevent knocking, while introducing a fuel injection retard for correction corresponding to the amount of the spark retard for correction so that in the stratified combustion mode of operation, fuel injection and spark can be timed to have a relationship allowing a spark to be provided when injected fuel flows in a vicinity of a spark plug. Thus in the stratified combustion mode of operation a spark retard for correction can be introduced to prevent an engine from knocking while misfire, poor exhaust emission, and the like can be prevented by introducing a fuel injection retard for correction.
Furthermore, in general, the double injection stratified combustion mode allows spark and fuel injection to be timed for correction over a wider range than the stratified combustion mode. Thus in the former mode knocking can be more readily reduced than in the latter mode. As such, in the stratified combustion mode of operation when an amount or result of correction introduced to prevent the engine from knocking exceeds a prescribed threshold value (e.g., the amount or result is limited by a correction guard value), then a decision is made that the knocking cannot completely be prevented in the stratified combustion mode, and the stratified combustion mode is switched to the double injection stratified combustion mode. Such knocking that cannot completely be prevented in the stratified combustion mode, can thus be handled.
Furthermore, in the double injection stratified combustion mode, in addition to a timing of spark a fuel distribution ratio (an amount of fuel injected at the intake stroke) and a timing of injection of fuel at the intake stroke can also be corrected to prevent an engine from knocking. Note that if the fuel distribution ratio (the amount of fuel injected at the intake stroke) is corrected, the amount can be reduced to a level disallowing self-ignition to prevent the engine from knocking, or, in contrast, the amount can be increased and heat generated as injected fuel vaporizes at the intake stroke can be utilized to reduce the temperature of a gas present in the cylinder to prevent the engine from knocking.
Furthermore the homogeneous combustion mode allows spark and fuel injection to be timed for correction over a further wider range than the stratified combustion mode and the double injection stratified combustion mode. Thus in the former mode knocking can be further more readily reduced than in the latter modes. As such, in the stratified combustion mode of operation if the detector detects that the engine knocks or in the double injection stratified combustion mode of operation if the engine knocks and the knocking is reduced as controlled, and as a result at least one of an amount or result of correcting the fuel distribution ratio, the timing of fuel injection at the intake stroke, and the timing of spark exceeds a prescribed threshold value, then the mode is switched to the homogeneous combustion mode. Such knocking that cannot completely be prevented in the double injection stratified combustion mode or (the stratified combustion mode), can thus be handled.
As disclosed in Japanese Patent Laying-Open No. 2004-003429, the knocking prevention controller corrects a timing of fuel injection as it exerts control to prevent knocking. It should be noted, however, that an in-cylinder air fuel mixture has a distribution (in concentration) that can vary with the timing of fuel injection. The air fuel mixture having a varied distribution results in a varied timing (or speed) of propagation of flame in the cylinder. The timing of propagation of flame in the cylinder that varies with location can cause the engine to knock. The publication, however, neither discloses nor suggests such a disadvantage in connection with the knock prevention controller disclosed therein. Thus there still exists the problem of the uncorrected timing of propagation of flame in a cylinder.